Power or voltage converter circuits are used to convert a voltage source to from one type of power (e.g., alternating current (AC) or direct current (DC)). Such AC source may include a generator or electrical power grid power source. Such a DC source may include a battery or DC power supply. Other types of power converter circuits include circuits that merely change the voltage level. For example, a DC voltage level may desirably be increased or decreased according to the needs of the load device. Generally, a power converter circuit provides an output voltage that has a different level than the input voltage.
Direct Current to Direct Current (DC to DC) circuits are a common type of power converter. DC to DC power converters storing input energy from a power source and then releasing that energy to an output load at a different voltage level, typically at a high frequency. DC to DC converter circuits are generally more power efficient than linear voltage regulators because they dissipate less power as heat. It is generally desirable to operate switched mode power converter circuits at higher frequencies as the component size and weight of such circuits can be reduced by, for example, eliminating low frequency transformers.
In recent years, with advance in power of high-frequency semiconductor element, high frequency characteristics and reliability in a field effect transistor (FET) are often used in such power converters. In particular, GaN-based semiconductors that contain GaN as a principal constituent are often being used because of its high saturation electron velocity and high voltage breakdown characteristics.
GaN-based switching devices, such as hetero-junction field effect transistors (FETs), are advantageous over other types of conventional FET solutions in that they posses higher power densities, higher breakdown voltages and lower on-resistances. GaN FETs are typically formed as normally ON depletion mode devices. For certain applications, such as power applications, normally ON devices can be less desirable than normally OFF devices such as standard FET devices because normally ON devices are often operated less efficiently than normally OFF devices and the drive circuitry for normally ON devices has conventionally been more complicated and expensive than normally OFF devices. Thus, it is known to configure a depletion mode GaN FET in a cascode configuration with a silicon FET (Si-FET) to improve efficiency.
GaN-HEMT/Si-FET cascode devices may be utilized in such power converter circuits but they are problematic because they often have a large voltage spikes or rapidly changing voltage levels (dv/dt). For example, the rate of change for the voltage can exceed 100V/ns. Control by common techniques is often found to be difficult in some applications. Large dv/dt is a problem in synchronous DC/DC converters, for example, because of EMI issues and driver incompatibility. In some circuits, a desired maximum voltage rate of change is 50V/ns. Accordingly, because of the advantages of GaN-HEMT/Si-FET cascode devices in high voltage high power converters, a need exists for reducing the large voltage spikes (dv/dt) resulting from high current levels flowing into the circuit capacitors and associated problems that may be experienced.
The use of the same reference symbols in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.